Bleach (sodium hypochlorite) is a commodity chemical that is used in numerous applications. The basic chemistry for manufacturing bleach is a matter of common knowledge in the fields of chemistry and chemical engineering. Chlorine in gas and/or liquid phase is allowed to react with a solution of sodium hydroxide (caustic) to yield aqueous sodium hypochlorite. While that basic chemistry may be considered rather elementary, and essentially common to all processes for the commercial manufacture of bleach, specific processes that have been described in patent literature differ in significant ways.
Each of the various known processes for the commercial manufacture of bleach may be characterized as either a batch (discontinuous) production process or a continuous production process. Each type of process may have its own particular advantages.
A continuous process that is properly controlled is more likely to be performed with higher production efficiency than a corresponding batch process, and hence is likely to be more economical than a batch process. However, the specific manner in which a continuous process is performed plays a significant role in the nature and quality of the resulting bleach product.
U.S. Pat. Nos. 4,428,918 and 4,780,303 each describes a respective continuous process for manufacture of concentrated (i.e. high-strength) sodium hypochlorite solutions. Sodium chloride (salt) is however also a product of the basic reaction, and its removal from the aqueous sodium hypochlorite product can improve both the continuous process and the resulting product. Neither of those processes removes all of the salt from the resulting product.
U.S. Pat. No. 4,428,918 describes the resulting product as an aqueous solution of sodium hypochlorite that is recovered at an hourly rate of 1775 kg. (kilograms) and that contains 257 g. (grams) of sodium hypochlorite and 94 g. dissolved sodium chloride per kg. of product. The process is said to create a suspension from which some salt is removed by a filtration apparatus through which an hourly fraction of 2051 kg. of the suspension is circulated. It is from that filtration apparatus that the aqueous solution of sodium hypochlorite is recovered, and it is said that the filtration apparatus separates out a cake containing, by weight, 80.1% sodium chloride and 19.9% sodium hypochlorite. The patent does not mention the presence of any sodium chlorate or excess caustic in the resulting sodium hypochlorite product.
U.S. Pat. No. 4,428,918 also does not quantify salt crystal size in the suspension, but it does observe that the economical removal of fine salt crystals is difficult. Crystal size is said to be increased by a recycling of the suspension through a heat exchanger that cools the suspension. It is also said that a point is eventually reached where further increases in crystal size are not obtained. The suspension is understood to exist throughout the column where crystallization is occurring and hence, there would be no zone of essentially crystal-free mother liquor.
U.S. Pat. No. 4,780,303 describes a continuous process where salt is crystallized from the suspension in what is described as a second stage, or crystallizer. A solution, that is obtained from a first stage and said to contain 14.5% sodium hypochlorite and 3.2% sodium hydroxide, is introduced into the second stage where it is reacted with chlorine to create an aqueous solution of sodium hypochlorite with salt present both in solution and suspension. The solution is subjected to mechanical agitation within the crystallizer while solution is being withdrawn from the top of the crystallizer, recirculated through an external heat exchanger, and re-introduced into the bottom of the crystallizer. A fraction of the solution that is drawn off from the top of the crystallizer is diverted from the heat exchanger to form the resulting bleach product which is said to comprise 25% sodium hypochlorite, 9.5% sodium chloride, and a slight excess of caustic (0.3%–0.8% by weight).
It is also said that no salt is precipitated in the first stage where chlorine is reacted with a caustic solution to create the solution introduced into the second stage. Crystallized salt is said to be removed from the second stage by precipitation, either continuously or intermittently, and is said to have a mean crystal size in the neighborhood of 400 microns (about 400 to 500 microns). The slight excess caustic is said to avoid the formation of sodium chlorate (NaClO3), but the patent is silent about any amount of sodium chlorate that the bleach product might actually contain.
Because U.S. Pat. No. 4,780,303 requires agitating the contents of the crystallizer to promote reaction, it would be understood that the solution drawn off from the top of the crystallizer would not be free of salt crystals.
Both patents recognize that certain batch processes can produce aqueous high-strength bleach from which significant amounts of salt have been removed. Various batch processes are referenced by cited patents, both domestic and foreign.
It is believed that a continuous process that is capable of consistently producing aqueous high-strength bleach with low concentrations of both sodium chloride and sodium chlorate with residual slight excess caustic would be beneficial to industry. A product that has even greater strength, and lower salt and chlorate concentrations, than those mentioned in U.S. Pat. Nos. 4,428,918 and 4,780,303 would be especially beneficial. The benefits reside both in the utility of the product and relevant economic factors.